Theory of Polarization of Molecular Line Radiation Excited by Electron Impact

The Percival-Seaton theory for the polarization of atomic impact radiation is extended to the case of diatomic molecules. Similar to the atomic case, it is shown that for excitation by electron impact involving a change of electronic angular momentum along the molecular internuclear axis, the threshold polarization can be determined from the symmetry of the total molecular wave function without a detailed knowledge of the inelastic cross sections. General expressions are developed for the cases of fine and hyperfine splitting, and explicit relations are given for the first two rotational levels of a ${}_{}{}^3{}_{}{}^{}\Pi _u^{}{}_{}{}^{}$ state undergoing a radiative transition to a ${}_{}{}^3{}_{}{}^{}\Sigma _g^{}{}_{}{}^{}$ state in the limit of fine and hyperfine separations being much larger than the natural linewidth. This theory is valid when the radiative state is populated by a direct interaction, and the radiative transition between rotational states is resolved.